This invention relates to high speed optical scanners. More particularly, this invention relates to high speed optical scanners for use with finite conjugates and visible, ultraviolet, and infrared light.
A basic compact video rate optical scanner (CVROS) system is described in detail in U.S. Pat. No. 4,538,181 and is shown in FIG. 11. The disclosure of this patent is incorporated herein in its entirety by reference. In this system, incoming radiation from an image is reflected off a framing mirror 20 through a meniscus lens 22, past a strip mirror 24 to an objective mirror 26. The radiation is then focused on strip mirror 24 and reflected toward the concave reflectors 28 of rotating scan disc 30, from which it emerges as a collimated beam. The collimated beam passes the strip mirror and is focused on detector 32 by means of a collector reflector 34. A two dimensional raster scan may be obtained by oscillating framing mirror 20 located at the aperture stop and synchronized to the scan disc.
The curved strip mirror 24 is the exterior surface of a cone formed with a 45 degree half-angle and an axis which is coaxial with the axis of rotation for scan disc 30 as indicated by dotted lines 36. Objective mirror 26 is a concave mirror with a spherical contour having a radius equal to the distance from the vertex of framing mirror 20. The reflected image off objective mirror 26 lies on a spherical surface which is intercepted by curved strip mirror 24. The radius of curvature of the image is one-half the radius of curvature of objective mirror 26 and is equal to the scan disc radius R.sub.D.
The CVROS scanner described in U.S. Pat. No. 4,538,181, however, suffers from significant limitations preventing its utilization in applications with finite conjugates and visible, ultraviolet, and infrared light. The CVROS scanner was originally developed for infrared applications. Consequently, all lens materials were made from material which transmits infrared radiation but not visible light, and are therefore inappropriate for applications in the visual spectrum.
For example, adaptation of the CVROS scanner for applications in the visible, ultra-violet or near infrared spectrums requires elimination of the germanium meniscus lens because of its spectral transmission limitations. This results in the need for a meniscus lens of a different material for the spectrum of interest, or to correct the spherical aberration by another means. A single lens is suitable only for essentially monochromatic applications. Broad spectral band applications would require an achromatic corrector lens which can correct chromatic aberration both on-axis and off-axis.
On-axis aberrations of the scanner are those which occur when the scan disc concave reflector is centered on the optical axis. Off-axis aberrations of the scanner are those which arise during the repositioning of the scan disc concave reflector as it rotates about the scan disc axis. With a perfectly symmetrical, monocentric system which includes the concentric curved image surface, the off-axis aberrations are identical to the on-axis aberrations. Thus, with respect to the off-axis imagery for applications which require a flat image, the well-corrected curved image must be transformed into a flat image while maintaining the high degree of correction present across the entire arc length of the curved image.
Adaptation of the scanner to finite conjugate applications also requires dealing with the field curvature problem which arises. As is illustrated in FIG. 12, the image surface 38 (or object surface depending on application) for which the scanner retains its concentric form is spherical. In FIG. 12, Rm designates the objective mirror radius, Rs the strip mirror radius, and Ri the radius of the image. The image and object conjugates of the objective mirror are designated s' and s, respectively. For many applications, it is required that the scanner image onto a flat surface. Further, for applications in which the imaging medium or art work are located on a rotating drum or flat bed transport, the scanner need only produce a flat line scan and not a flat two-dimensional area scan.